A) Field of the Invention
The present invention relates to an apparatus using liquid crystal, and more particularly to a liquid crystal display unit.
B) Description of the Related Art
A vertical alignment liquid crystal display unit has a phenomenon that liquid crystal molecules are slanted in a horizontal direction because of an oblique electric field generated at edge portions of upper and lower electrodes having different shapes. This phenomenon occurs near at a threshold voltage. A vertical alignment liquid crystal display unit operates often in a normally black mode by using a pair of polarizer plates cross-Nicol disposed. An area of a liquid crystal display unit where this phenomenon occurs enters an optically transmissive state. In simple matrix driving applying an OFF voltage, a contrast ratio is lowered because of this optical through.
Japanese Utility Model Unexamined Publication No. HEI-7-39076 proposes to dispose a metal light shielding mask (black mask) covering pixel edge portions in order to prevent optical through (optical leakage) at pixel edge portions in a display OFF state.
A vertical alignment type liquid crystal display unit has a liquid crystal molecule alignment direction in a liquid crystal layer vertical to the substrate in the absence of applied voltage and has a very good black level in the absence of applied voltage. Of a liquid crystal cell constituting a vertical alignment type liquid crystal display unit, one or both upper and lower substrates have optical compensation plates having negative optical anisotropy disposed at proper positions so that the display unit has very good visual angle characteristics.
Orientation types of a vertical alignment type liquid crystal display unit include a mono-domain orientation formed by a rubbing process or the like and a multi-domain orientation formed by generating an oblique electric field by forming an opening in a pixel electrode to align liquid crystal molecules in a plurality of directions in the same plane parallel to the substrate.
Orientation control of a mono-domain vertical alignment type liquid crystal display unit is performed so that the in-plane alignment direction in a liquid crystal layer becomes uniform irrespective of whether a voltage is applied or not. In the vertical alignment type, liquid crystal molecules are slanted in a state parallel to the substrate in the presence of applied voltage. If liquid crystal molecules are perfectly vertical to the substrate in the absence of applied voltage, orientation defects are likely to occur which cause partial orientation disturbance of liquid crystal molecules in the presence of applied voltage. In order to avoid this, a pretilt angle is set so that liquid crystal molecules tilt slightly from the vertical direction relative to the substrate in the absence of applied voltage.
A vertical alignment liquid crystal display unit subjected to a rubbing process is proposed, for example, in JP-A-2005-234254.
In driving a mono-domain vertical alignment type liquid crystal display unit at a high duty ratio like dot matrix display, a drive voltage is determined based upon a transmissivity. In this case, an effective value of an OFF voltage is determined from an effective value of an ON voltage and a bias ratio. Under the high duty ratio driving conditions, the OFF voltage is in some cases higher than a threshold voltage at which liquid crystal molecules start falling. In this case, therefore, a transmissivity of liquid crystal changes even in the presence of applied OFF voltage, and optical through may be formed in a display unit. This optical through is desired to be avoided because a contrast ratio is lowered.
It is known that a change in voltage/transmissivity is made steep near the threshold voltage in order to prevent the optical through. One method for this is to perform a large pretilt angle orientation process of setting liquid crystal molecule angles near vertical relative to the substrate in order to raise the threshold voltage at which liquid crystal molecules start falling. Another method is to increase a so-called retardation of a liquid crystal cell.
An oblique electric field may be formed at edge portions of a display pattern of a liquid crystal display unit between upper and lower electrodes. Even if the above-described optical through preventing methods are incorporated, liquid crystal molecules applied with the oblique electric field start falling at a voltage lower than that at liquid crystal molecules in the central area of a display region so that optical through is observed.
Optical through causes not only a lowered contrast ratio but also degraded visual angle characteristics.
A liquid crystal display unit for segment display is used for numerical display with seven segments and mark display having a particular shape, and the like. Techniques of forming a black mask serving as a light shielding film in the whole background area of a segment display pattern are disclosed, for example, in JP-A-HEI-5-281559, JP-A-2000-250024 and JP-A-HEI-5-2161. HP-A-HEI-5-2161 describes that a black mask is formed in the background area of a dot display pattern even for dot matrix display.
As described in JP-A-HEI-5-2161, one of the effects of a black mask is that it is possible to suppress optical through to be caused by an oblique electric field generated near at edge portions of a display pattern. This optical through is likely to be formed particularly in a vertical alignment type liquid crystal display unit.
However, as a black mask is formed in the background area, there occurs a phenomenon (this is herein called crosstalk caused by black mask) that even if black display is given to the display pattern, the display pattern is viewed dim.
The crosstalk caused by a black mask occurs because a transmissivity of a display pattern cannot be set as compatibly low as that of the black mask, in any display mode of twisted nematic (TN), super twisted nematic (STN), vertical alignment (VA) and the like. This transmissivity difference becomes conspicuous as a view angle is slanted.
With reference to FIG. 35, description will be made on a schematic example of crosstalk caused by a black mask. FIG. 35 is a schematic plan view showing a display state of a 7-segment display unit of three digits. A black matrix is formed in the whole background area of the 7-segment display unit of three digits, and the background area is black. The 7-segment display unit of three digits displays “321” in white. In the 7-segment display unit, segments (indicated by hatched lines) in black display have a higher transmissivity than that of the background black mask forming area, and are viewed as if the segments are displayed dim.